Non-Foster matching circuits have been proposed in the prior art. See FIGS. 1(a) and 1(b) which are discussed below and J. G. Linvill, “Transistor Negative Impedance Converters,” Proc. IRE, vol. 41, June 1953 and Stephen E. Sussman-Fort and Ronald M. Rudish, “Non-Foster Impedance Matching of Electrically Small Antennas,” IEEE Transactions on Antenna and Propagation, Vol. 57, August 2009.
Many people have utilized circuits in communications systems having electrically small antennas (ESA) to try to match the antenna to the receiver (or transmitter) of the communication system. Electrically small antennas have a smaller size than a full size antenna. A full size dipole antenna has a size of
  λ  2while a full size monopole antenna has a size of
  λ  4and a full size loop antenna has a size of
  λ  3(where λ is the wavelength of the frequency where the antenna is resonant). The term “match” in this context refers to an impedance match between the antenna and its transmitter (or receiver). If there is a mismatch between an transmitter (for example) and its antenna, then energy from the transmitter is reflected back into the transmitter, which, if large enough, can either damage the transmitter or cause its protection circuits to take it off line. As the electrically small antenna is made to be less and less the size of its equivalent full size antenna, the match becomes worse and worse. So matching circuits are devised which improve the impedance mismatch between the antenna and the transmitter (or receiver). Electrically long (or big) antennas also produce a similar mismatch but electrically small antennas are of more commercial (as opposed to theoretical) interest due to designers making cell telephones (for example) small without the need for a protuberance for the antenna.
FIGS. 1(a) and 1(b) are schematic diagrams showing two different matching circuits for use with electrically small antennas, where the circuit of FIG. 1(a) employs a passive matching circuit PMC (which does not utilize active circuit non-Forster techniques) that directly connects the electrically small antenna (such as a6 inch monopole antenna) to a receiver having a 50 ohm input impedance and FIG. 1(b) tries to better the match by using a non-Foster matching circuit for generating a negative capacitance value to improve the impedance match between the the electrically small antenna and the receiver. In FIG. 1(b) the non-Foster circuit synthesizes a negative capacitor which is used to cancel the positive capacitor effect of the electrically small monopole antenna.
In a passive matching circuit a positive inductor can be used to offset the effective positive capacitance of the electrically small antenna but the Q of the tank circuit formed by the positive inductor and the positive capacitor of the electrically small antenna means that the combination of the positive capacitor of the electrically small antenna and an inductor has a relatively narrow bandwidth over which a suitable match occurs. Using a non-Foster matching circuit to cancel the positive capacitor of the electrically small antenna delivers a wider bandwidth match with a possibility of achieving higher gain than is possible with mere passive matching. However, the non-Foster circuit architectures used in the aforementioned prior art matching circuits often suffers from (i) inferior noise performance and (ii) stability issues, both of which issues have limited the use of prior art non-Foster circuit architectures in many applications.
FIG. 2 shows a non-Foster circuit using bipolar transistors and resistors (see also the article by Stephen E. Sussman-Fort and Ronald M. Rudish, referenced above). The circuit noise performance of this circuit is determined by the biasing resistors R1 and R2. Due to a >50 ohm resistance of R1 or R2, they contribute significant noise and the circuit demonstrates a high noise figure. In addition, traditional non-Foster circuits tend to oscillate, which mandates stable design methods.
J. G. Linvill has disclosed many non-Foster implementation architectures. See J. G. Linvill, “Transistor Negative-Impedance Converters,” Proceedings of the IRE, vol. 41, pp. 725-729, 1953. FIGS. 3(a) and 3(b) depict two examples of same. However, these circuits still have a high noise figure and thus cannot be utilized reasonably in low noise applications. Also, as mentioned above, traditional non-Foster circuits tend to oscillate.
There are many non-Foster circuit architectures for synthesizing a negative capacitance or a negative inductance and most of them leverage positive feedback to gain that desired effect. In order to realize the required positive feedback network, bias networks are needed for supporting the required active circuitry. Unfortunately, existing bias circuits used in prior art non-Foster circuits are either realized by resistors or current sources, both of which contribute significant noise that dominates the noise performance of the resulting non-Foster negative capacitor or negative inductor. FIG. 4 shows a typical non-Foster circuit, where the noise from the bias circuit contributes 90% of the total noise of the non-Foster circuit (as determined through circuit simulation techniques). To be useful in receiver applications, it should now be apparent to the reader that a low noise bias circuit is needed to realize a low noise non-Foster circuit.